The present invention relates to a rubber composition for the tread of high performance tires having a large hysteresis loss, a high heat resistance and a remarkable grip.
As highway networks have been increasingly developed in recent years, various performance characteristics of automobiles, for example the high-speed performance, have been progressively improved, and in accordance with this, the demand has been increasingly strongly made that there should be provided high performance tires which can exhibit an improved stability with respect to such as the steering performance, the cornering performance and the braking performance at the time when automobiles mounting the tires are run at a high speed.
Also, in connection with racing tires which are put to service in a condition where the limit performances of high performance tires in general are competed with one another under severer service conditions than those under which high performance tires in general are put to service, as motor sports have grown to be increasingly popular of late there has grown a demand for further improving the tire performance, and today it is sought for to attain an improvement relating to the steering performance and the braking performance at critical times in service of tires.
To improve the performance characteristics required of high performance tires in general and racing tires as above, it is effective to improve the grip performance of the tires relative to road surfaces. For example, by increasing the hysteresis loss of the part (a rubber composition) forming the tire tread that contacts the road surfaces, it is possible to enhance the grip performance of the tires.
Then, to obtain a rubber composition having an increased hysteresis loss, conventionally it has been generally practiced to make use of an emulsion-polymerized styrene-butadiene copolymer rubber having a high styrene content with a high glass transition temperature (Tg).
In connection with styrene-butadiene copolymer rubbers, however, although their hysteresis loss can be increased as the styrene content therein is increased, their blowout resistance characteristic becomes lowered at the same time. Particularly, styrene-butadiene copolymer rubbers produced by an emulsion polymerization have such a broad molecular-weight distribution that they tend to show a considerably low blowout resistance, and they often fail to stand practical use in or for the rubber composition for racing tires which are used under a relatively severe service condition.
It has been also proposed in order to raise the hysteresis loss to blend in a diene rubber a butyl rubber or a halogenated butyl rubber. However, rubber compositions made with use of such blended rubbers tend to be poorly co-vulcanizable and poor in the breaking strength, and when they are used in or for high performance tires and racing tires which are subjected to severer service conditions than the high performance tires, a problem is posed that the resulting tires are prone to undergo a considerable abrasion and a blowout.
As indicated above, it is the status of the art in which there has not been a rubber composition provided, which is useful for tires which have a grip performance of such a high level as being required to attain in the cases of tires for racing cars including rally racing cars and of which it is unnecessary to concern about the heat resistance (blowout resistance).